How to Tune a Q-Jet (basic) by Lars Grimsrud SVE Automotive Restoration Musclecar, Collector & Exotic Auto Repair & Restoration Broomfield, CO
Rev. New 10-4-01
This tech paper will discuss basic set-up and tuning of QuadraJet carbs for optimum street performance and drivability. The procedure outlined here differs from other I have seen, and is based on my years of experience doing this work in the quickest, least painful, most economical way. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.
Overview The Rochester QuadraJet, in its various forms and configurations, has been used by various GM Divisions for various applications since the mid sixties. The last passenger car version of the carb appeared as an ECM-controlled carb in 1981. The Q-Jet is a highly versatile, tunable carb that will provide outstanding performance and reliability once set up correctly. This paper will discuss the tuning and setup, and will provide you with adequate data to make good decisions when jetting and adjusting the carb. This paper will not discuss basic rebuilding sequences, nor will I discuss operations involving machining operations and other severe alterations to the carb. There are many books on the market that deal with these subjects in depth. Rather, I will describe the various systems, their purpose, and a good tuning sequence to help you get each system and parameter set up correctly in the easiest way possible. QuadraJet carbs have three basic tuning variables, and these get people all confused: Primary Metering Jet, Primary Metering Rod, and Secondary Metering Rod. Attempting to cure problems by tuning the wrong variable results in lots of frustrations for tuners and car owners. These systems at times overlap in their operation. Not only does each system need to be properly tuned, but its timing and “overlap” with other systems is critical to proper performance. When tuning, we think of each of these variables as controlling a different operating range: The primary jet size determines the fuel mixture at Wide Open Throttle (WOT). The primary metering rod determines fuel mixture at cruise speed and determines responsiveness of the idle mixture screws and off-idle performance. The secondary rods are a high-rpm compliment to the primary side, and are used for final “tweaking.” On a Q-Jet, we see that we can control the fuel mixture throughout the operating range. This is different from a Holley: A Holley has a given main jet size which meters fuel throughout the rpm range, including cruise. At WOT, the power valve unseats, and opens a fixed orifice, dumping a fixed amount of fuel in addition to the main jet. Crude, but simple and effective. The Q-Jet meters fuel through the main jets. Metering rods, suspended from a power piston, “plug off” part of the area of the main jets by being inserted into the jets. These rods have a “fat” diameter and a “skinny” diameter: The number stamped into the side of every metering rod is the “fat” diameter indicated in thousands of an inch. This part of the rod is pulled into the main jet at cruise, at idle and at other high-vacuum operating conditions (light throttle). It produces a lean operating condition for good fuel economy and good throttle response. When engine vacuum is lost, indicating a high-power condition, the rods are pushed out of the jets by spring pressure, and only their “skinny” tips, or power tips, remain in the jets. This richens the fuel mixture up for peak power. All primary metering rods have the same power tip diameter (.026”). This fact is crucial to remember when tuning: Primary metering rod sizes have no effect on WOT performance.
Identification Q-Jets are identified by a number stamped into the Float Bowl casting on the driver’s side of the car just above the secondary throttle linkage. If the carb is a Carter manufactured under license froom Rochester, the number will be inside a round metal foil tag on the driver’s side of the carb just above the primary throttle linkage.
Metering Area WOT fuel mixture is controlled only by the main jet size. Performance at cruise and at idle is then controlled by the rods. We can establish each of these mixtures independently of the other by knowing and understanding the concept of Metering Area. Jet and rod sizes are always referred to by their diameter in thousands of an inch. But fuel flow doesn’t “see” diameters: The fuel “sees” the total metering area. So we must convert the diameter into a resulting area. We remember that the formula for area is πr2. Thus a jet with a diameter of .070” has a metering area of: Radius = ½ diameter Radius = .035” π.0352 = .00384” Thus, the metering area of a #70 jet is 3.84 thousands of a square inch. But wait! There is a rod inserted into the jet, so we must subtract the area of the rod. Let’s say we have a #40 rod in that #70 jet. The area of the rod is: Radius = ½ diameter Radius = .020” π.0202 = .00125” Thus, the area that a #40 rod “plugs off” is 1.25 thousands of a square inch. The resulting metering area of the #70/#40 combination is thus 3.84 minus 1.25. The total metering area is 2.59 thousands of a square inch. This is the metering area of this rod/jet combination with the rod fully inserted in the jet. In other words, this is the metering area at cruise speed and at idle. To see the metering area at WOT, we know that all rods have a .026” diameter power tip. So we run the same calculation for a .026” diameter rod inserted in the jet. It is these numbers that we will use in all comparisons when making jet changes. We will use these numbers also to look at the percent differences in jet changes. So that you won’t need to run around with a calculator, my Carb Listing in Table 1 shows the metering areas for every carb listing at both cruise and at WOT. The number is the metering area in thousands of an inch for a single jet/rod in the carb. This number is effectively how rich/lean the carb is really jetted, and you can directly compare these numbers to see how the various carbs were set up by the factory. By dividing one area into another area, you can see the percentage difference in the jetting. Figure 2 is a table showing you what the metering area is for every possible jet/rod combination. Each grouping of jets starts off with the rod power tip diameter of .026” so you can see the WOT metering area of that jet size. It then jumps to the first usable rod size. Tech Tip #1 Before you go trying to fix all the errors of the previous carb tuner, set your carb up to the stock spec for your carb part number. A carb jetted and set up to its stock specs will usually run pretty good on just about any application, and this gives you a good starting point. From there, you can start doing refinements as outlined in this paper. The carb number on a Q-Jet is usually stamped into the bowl casting on the driver’s side of the carb in the area above the secondary throttle shaft. The number starts with either “70…” or “170…”. If the carb is a Q-Jet manufactured under license by Carter, it will sometimes have the carb number stamped into a foil circle on the driver’s side of the bowl just above the primary throttle shaft.
The carb listing (Figure 1) is a partial listing of popular Pontiac and Chevy Q-Jets that I have compiled over the years. It is not a complete listing of every carb used by Chevy or Pontiac. Most notably, I have very few of the truck carbs listed, yet there are many truck carbs running around on passenger cars. Tech Tip #2 What has a greater effect on performance: primary or secondary jetting? I constantly see people swapping around secondary rods, trying to get the best performance out of their cars. The secondary rods are very easy to change, and since the secondaries are so BIG, the secondary metering has to be the most important, right? Wrong. Most Q-Jets are 750 cfm carbs. This is more airflow than most small block engines can ever handle. Yet, GM used Q-Jets on everything from Overhead Cam 6-cylinder Pontiacs and Corvair 6-cylinders, to 500 cube Caddys. How? The secondary airvalve on the Q-Jet effectively makes the Q-Jet a variable-cfm carb. The spring windup of the airvalve combined with the bleed-off of the choke pulloff diaphragm allow the secondaries to open only as much as the engine can handle. Thus, if the engine can’t handle all of the cfm, the secondaries simply don’t open all the way. The primary side, however, is used throughout the rpm range. It is always in use, and provides the metering for the majority of the power produced by the engine. Let’s look at the scenario: You’re at the stoplight. You bring the rpm up slightly against the torque converter – 1500 rpm. You’re on the primary side of the carb only, and this is what is producing all of your torque right now. The light changes, and you put the pedal to the metal. All of your torque at launch is being produced by the primaries only, as the secondaries don’t see enough airflow to open. The rpm comes up quickly: 2000, 2500, and now the secondaries might be starting to crack. Almost all of the air is still passing through the primaries, and the secondaries are now starting to compliment it just a tad. 3000, 4000 rpm, and the secondaries might be half-way open. The primaries are still providing most of the airflow and metering. 5000, 5500 and you hit redline just as the secondaries hit about ¾ open. Second gear, your rpm drops, partially closing the secondaries back up, and you’re back to sucking the majority of the air through the primaries once again. So we see, the secondaries provide only a compliment to the primaries. The primaries provide the vast majority of the fuel metering, and primary jetting is absolutely the most critical to proper performance. You cannot compensate for poor primary jetting by rejetting the secondaries. So we are going to concentrate on jetting the primary side for peak performance, and then we will set up the secondary side to provide a proper compliment to the correct primary jetting. Tech Tip #3 How can you tell if an off-idle stumble is caused by a lean or a rich condition? A carb running rich, as well as a carb running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill bit that’s just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don’t jam the pin right up against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again. With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won’t have any trouble starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change whatsoever, you have a non-functional accelerator pump. To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition. You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10% increments using the Figures provided in this paper. Tech Tip #4 How can you tell if your power piston spring is too stiff and not allowing the power piston to “seat” at idle?
If your engine does not produce enough manifold vacuum at idle and/or cruise (due to a lumpy cam or other engine parameters), it is possible that the power piston is not being pulled all the way down to its seated position due to the power piston spring being too stiff. The result is that the car will run very rich at idle, and the idle mixture screws will have little effect or response. Idle speed may also “float,” with idle speed starting high and gradually decreasing until the engine stalls due to the engine getting “loaded up.” There will typically be a puff of black smoke out the tailpipes when you “flick” the throttle. To test for this, pop the top off the carb, remove the power piston/rod assembly, and remove the power piston spring from its bore. Re-install the rod/piston assembly without the spring and put the carb back together. The carb will now run in the full-lean condition all the time. If this clears up the idle, improves idle mixture screw response, and eliminates the black smoke when you flick the throttle, you need to install a softer spring. Edelbrock has a complete power piston spring assortment available. Tech Tip #5 How can you tell how stiff the power piston spring needs to be, and how can you tell one spring from another? If you have a few springs of various kinds laying around, it is not readily apparent which spring is stiffer than another. You can arrange them and order them from softest to stiffest as follows: Using your carb, or a junk float bowl from another carb, as a testbed, remove the carb air horn (the “top” of the carb) and remove the power piston and its spring. Remove the primary metering rods from the piston. Now, drop a spring into the power piston bore and install the piston. Find a Phillips screwdriver, and place the handle of the screwdriver on top of the power piston with the shank of the screwdriver pointing straight up. Use a screwdriver that is light enough to NOT compress the power piston and its spring, but close. Now, drop flat washers onto the shank of the screwdriver and keep stacking them up until the piston compresses the spring and seats in the bore. Count the number of washers it took to compress the spring and label the spring as a “6-washer spring,” for instance. Do the same with the other springs you want to test. You’ll end up with a comparative rating of springs, like “4-washer,” “6-washer,” or “10-washer” springs. You now know exactly how to arrange them from softest to stiffest. But which one should you use? You’ll need a junk Q-Jet float bowl for this test, and you’ll need to have your engine in running condition. Using a stripped down, bare Q-Jet float bowl, you’ll notice that there is a hole in the bottom of the bowl right underneath the power piston bore. This is the vacuum hole that applies manifold vacuum to the power piston. Hook up a long vacuum hose to a manifold vacuum source on your engine. Now, install a power piston spring from your arranged spring selection into the piston bore and install a power piston on top of the spring. Start your engine, and stick the end of the vacuum hose onto the hole in the bottom of the stripped down float bowl. With the engine at idle, the vacuum applied to the bowl should immediately pull the power piston down against the spring pressure and seat the power piston firmly in its bore. If the piston does not fully seat, you need a softer spring from your arranged spring selection. If you have an automatic, put the transmission in “drive.” Make sure the power piston stays seated. If you really want to do some testing, you can string the vacuum hose into the car, and with an assistant, drive the car around and observe under what conditions the power piston starts to unseat: While you drive, have the assistant stick the vacuum hose onto the bottom of the bowl, and observe what the piston does under various engine loads. Make sure you have a spring that’s stiff enough to make the piston pop up when your engine is under load, yet soft enough to keep the piston fully seated at idle, at cruise and under light acceleration. This makes for some really fun testing, and the results will pay off in a precisely matched power valve spring for some outstanding throttle response. Of course, if you buy the power piston spring assortment kit from Edelbrock, the springs will be identified and labeled as to their vacuum rating. Select and use a spring with a rating about 1.5” to 2” lower than the idle vacuum of the engine (in drive). Tech Tip #6 The idle metering circuit on a Q-Jet is not an independent, stand-alone circuit. The idle mixture screws in the throttle plate receive their fuel through the main metering jets. Thus, a change in the main metering circuit (jets and/or rods) will affect the idle circuit. The idle mixture screws cannot meter more fuel than the main jets/rods will allow. Thus, if your Cruise Metering Area jet/rod combination is too lean, you may find that your idle mixture screws are ineffective. If your idle surges, is rough & unstable, and adjusting the screws seems to make no difference (but you can kill the engine by turning them all the way in), chances are good that your cruise metering area is too lean. You can verify this by running your mixture screws out to the point where additional turns have no effect on idle. Then cover the choke area of the carb with your hand. If idle speed & quality increases as you restrict the air flow, your jet/rod combination is too lean.
Procedure Here is my recommended sequence and procedure for doing a basic Q-Jet set-up: 1. Set the float level. You’ll be amazed how many people try tuning a Q-Jet without ever checking the float level. An incorrect float level can give you all kinds of symptoms and problems, so get this one set right off the bat. You have to pull the top of the carb off to set the float level. With the top removed, remove the big phenolic spacer that covers the area around the needle/seat. Hold the float hinge clip firmly seated and push down lightly on the float where it contacts the needle. Measure from the top of the float bowl to the top of the float at the rear edge of the float. Float level should be .375” - .400” for a street-driven car. Adjust by removing the float and bending its lever arm. Never raise the float level by forcing the float against the needle/seat to bend it – this will damage the needle. 2. Determine main jet size. If you have a stock engine, always start with the stock jet size and work from there. If you have the typical street modifications like headers, good exhaust system and a free-flowing intake, you can start with a main jet size 2 sizes larger than stock. Since we want to work on the primary side only, we don’t want the secondaries interfering with the jetting process. Chevy Q-Jets have a secondary lockout lever on the passenger side of the carb right at the secondary throttle shaft. This lever is actuated by the choke linkage, and prevents the secondaries from opening when the engine is cold. I call this the “primary jet tuning lever.” Use a piece of wire or string to engage the lever with the secondaries so that the secondaries cannot be opened. You now need to find a short flat stretch of road to test drive the car. You need to be able to measure time-to-distance and/or speedat-distance. I usually find a repeatable stretch of road about 300 feet long. This gets me through 1st gear and into 2nd. Make two or three runs on the car through this stretch and make note of time and speed to distance. Also note the seat-of-your-pants feel of the car (it’s going to feel pretty slow with the secondaries locked out…). I recommend making jet changes in less than 10% increments. Go to Figure 2 and determine your WOT metering area for your current jet size. This will be the metering area of the jet with the .026” rod. With this number, go to the Jet % Change Chart and find the closest metering area match in the left vertical Metering Area column (Use the “Area” column and not the Jet Size column. The Jet Size column can only be used on carbs that do not employ a metering rod, such as Holley and Weber.). Follow the row across until you get into the “green” zone and find the closest number to 10%, but not greater than 10%. Now go straight up until you get to the new metering area number. This is your target. Take this number and go back to Figure 2 and find the closest jet size that will produce this metering area with a .026” rod. This is the first jet size you want to try, and this will increase your fuel mixture by the percentage indicated in the chart. Now, to keep your off-idle mixture unaltered, you also need to check your cruise metering area. Go to Figure 2 and find your old main jet & rod combination. Note the resulting metering area for this combination. Now, go to your new main jet size that you’re going to be using and find the rod needed to produce the same cruise metering area you had before. Use this rod with the new jet. By doing this, you are now changing only 1 parameter at a time: WOT mixture only. Idle, off-idle, and everything else is now unchanged, and you will be able to see the results from the mixture change at WOT only. With the secondaries still locked out, run the car 2 – 3 times down the same stretch and record results. If the numbers get better, you’re going the right way with the main jet size. If the numbers are worse, you need to make changes to the lean side instead of rich. Repeat this operation until you determine the main jet size that produces the best numbers. On many stock cars, you may be surprised to learn that you end up with the stock jet size. You have now optimized main jets. 3. Determine main metering rod size. NOTE: There are two different “series” of primary metering rods. Q-Jets up through 1974 (the “4MV” series carbs) use the early series rods, also known as the “single taper” rods. 1975 and later Q-Jets (the “M4M” series carbs) use the second series rods, also known as the “double taper” rods. Not only do the rods differ in their taper design, but they are different lengths. You cannot interchange the two different rod series.
When switching main jets around, you were also swapping out metering rods to keep the cruise metering area unchanged. You did this to make sure that your off-idle throttle response remained unchanged so that the throttle response off idle did not affect the tuning results from the main jet re-sizing. Now, with your new main jets, your cruise metering area is exactly the same as it was before, but that’s not to say it’s right. There are several indicators of correct cruise metering area. First, check out Tech Tip #5 regarding the idle circuit. This is a good indication of a lean condition. But here’s another good indicator of correct cruise metering area: A Q-Jet, when set up with the correct metering rod for cruise & idle, will produce a slight hesitation upon acceleration if the accelerator pump is disconnected. Using a small pin punch or a finish nail, carefully knock out the roll pin securing the accelerator pump arm to the top of the carb. I do this with the engine running so I don’t have any trouble starting the engine without the accelerator pump. With the pump disconnected and with the engine running in neutral, “flick” the throttle just a little. If the engine actually feels more responsive with the pump disconnected, your cruise metering area is too rich, and you need to install a fatter set of rods. If you get a severe stumble, or if the engine dies, you’re on the lean side and need smaller rods. When the rods are correct for the jets in use, you will get a slight hesitation when the pump is disconnected. Once you have set the rod size up like this, verifying both the idle as shown in Tech Tip #5 and using the disconnected accelerator pump, a road test is in order. If the car is a little “flat” on light acceleration, or if it has a slight “surge” at steady cruise, you need to richen up the metering area slightly. If it is smooth and responsive on light acceleration, and feels smooth at cruise, you have the rod size nailed down. Again, use the charts to keep all changes limited to 10% at a time. This will prevent you from “over-shooting.” Remember, with the main jet size determined, your rod sizing is affecting idle, off-idle, light acceleration, and cruise. In most cases, when there are problems with stumbles, poor idle, and surging at cruise, the rods are too big and are causing a lean condition. On the other hand, if the rods are too small, causing a rich condition, the throttle will feel “lazy” or “slow” when you rev the engine, and you may get a puff of black smoke when you “flick” the throttle. Correct rods will produce crisp, clean and instant throttle response. 4. Determine secondary rod size. You are now finally ready to unlock the secondaries. But before you start changing the rods, you want to get the secondary opening rate set up. This is determined by the spring windup. It is a very common “speed trick” to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries. The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening. Now, you need to adjust the secondary rod hanger height. You’ve read all about the different letter numbers for the secondary hangers, and how a “Y” hanger will make your car faster than an “M” hanger or whatever. Fact is, you can bend and adjust any hanger to any hanger height you want, so it doesn’t make a heck of a lot of difference what hanger you choose to use. Just get it set up right: With the secondary airvalve held wide open and the secondary rods pulled all the way up, measure the distance from the top of the rear wall of the choke horn to the secondary rod hanger hole in the hanger. This distance should be 41/64”. Bend the hanger to adjust – you have to adjust each of the two sides independently. You now have a “performance” rod hanger. With this set, you can now play with secondary metering rods. A common speed trick mistake is to always install thinner (richer) secondary rods. Some engines and carbs will produce a secondary “lag” if the rods are too thin. On about half of the engines I work
on, I obtain better performance by installing fatter “non-performance” rods. Again, a quick road test is the only way to set this up, so go back to your 300-foot stretch and make a few runs with rods both richer and leaner. Once you have found the rods producing the smoothest secondary transition and the best numbers, you can start unwinding the secondary airvalve spring. Relax the spring tension in 1/8 turn increments until the car stumbles on acceleration, then tighten up 1/8 turn again. You have now determined the quickest secondary opening rate that your engine can handle, and your secondary mixture is set. Note that secondary metering rods come in three different tapers: long tip, short tip, and medium tip (see Figure 3). Most of the available after-market metering rods have the long tips, and these will produce a full-rich mixture upon the slightest opening of the secondaries. Many street engines will produce better performance by using the short tipped rods. A short tipped rod does not allow a full-rich mixture until the secondaries are opened quite a ways, keeping the mixture a little lean initially. This can produce smoother and crisper performance in many applications. Next time you see a junk Q-Jet laying around, make sure you yank the rods and jets out of it: many old truck carbs have some really good short-tipped secondary rods in them. Figure 3 lists all the secondary rod letter codes, part numbers, and measurements.
Parts If you don’t have a stash of used Q-Jets in your basement to rob jets and rods out of, you can get parts from Edelbrock. Youlocal parts store should be able to order them for you. Following is a partial listing of Edelbrock Q-Jet parts and part numbers: Primary Metering Rods (pairs) for 1974 & earlier: .035” .037”
#1936 #1937
.039” .041”
#1939 #1942
.043” .045”
#1944 #1946
CL
#1954
.076” .077”
#1976 #1977
Primary Metering Rods (pairs) for 1975 & later: .048” .050”
#1941 #1943
.052” .054”
#1945 #1947
Secondary Metering Rods (pairs) for all years: CC CE
#1950 #1951
CK AY
#1952 #1953
Primary Metering Jets (pairs) for all years: .068” .069” .070” .071”
#1968 #1969 #1970 #1971
.072” .073” .074” .075”
#1972 #1973 #1974 #1975
Questions, Comments & Technical Assistance If you have questions or comments regarding this article, or if you notice any errors that need to be corrected (which is quite possible since I’m writing this from memory…), please feel free to drop me an e-mail. Also, if you need any technical assistance or advice regarding this process, or other maintenance issues, feel free to contact me:
[email protected]
Figure 1: Carb part number listing & stock jetting Carb #
7027262 7027263 7028262 7028263 7028267 7028268 7028268 7028270 7028270 7028273 7028273 7028274 7028275 7029263 7029268 7029270 7029273 7037262 7037263 7037271 7040263 7040264 7040267 7040268 7040270 7040273 7040563 7040564 7040567 7040568 7040570 7040573 7041262 7041263 7041264 7041267 7041268 7041270 7041273 7041273 7042262 7042263 7042264 7042270 7042272 7042273 7042273 7042274
Application
Pont 67 400 AT & MT GTO Pont 67 400 MT w/o A.I.R. Pont 68 400 STD AT Pont 68 400 MT GTO Pont 68 400 H.O. MT GTO Pont 68 400 & 400 H.O. AT GTO Pont 68 GTO Best Tuned Condition Pont 68 400 RAM AIR AT after Jan 68 Pont 69 400 AT RAM AIR III Pont 68 400 RAM AIR MT after Jan 68 Pont 69 400 MT RAM AIR III Pont 68 400 AT EARLY RAM AIR Pont 68 400 MT EARLY RAM AIR Pont 69 400 MT GTO Pont 69 400 AT GTO Pont 69 400 AT RAM AIR IV Pont 69 400 MT RAM AIR IV Pont 67 400 AT w/A.I.R. GTO Pont 67 400 MT w/A.I.R. GTO Pont 67 400 RAM AIR after 6 Feb 67 Pont 70 400 MT FEDERAL GTO Pont 70 400 AT FEDERAL GTO Pont 70 455 MT FEDERAL GTO Pont 70 455 AT FEDERAL GTO Pont 70 400 & 455 RAM AIR AT GTO Pont 70 400 & 455 RAM AIR MT GTO Pont 70 400 MT CALIFORNIA GTO Pont 70 400 AT CALIFORNIA GTO Pont 70 455 MT CALIFORNIA GTO Pont 70 455 AT CALIFORNIA GTO Pont 70 400 & 455 RAM AIR CALIF. Pont 70 400 & 455 RAM AIR MT CALIF. Pont 71 455 AT GTO Pont 71 400 MT GTO Pont 71 400 AT GTO Pont 71 455 H.O. MT GTO Pont 71 455 H.O. AT GTO Pont 71 455 AT RAM AIR Pont 71 455 MT RAM AIR Pont 71 455 H.O. Best Tuned Condition Pont 71 455 AT CALIFORNIA Pont 72 400 MT GTO Pont 72 400 AT CALIFORNIA Pont 72 455 H.O. AT Pont 72 455 AT GTO Pont 72 455 H.O. MT Pont 73 455 S.D. MT Early Pont 72 400 AT FEDERAL
Main Jet
Main Rod
Spring
Sec. Rod
70 70 73 72 72 73 71 72 72 72 72 73 72 71 71 69 69 70 70 70 71 70 71 71 70 70 68 68 70 69 67 67 71 75 71 73 74 74 73 72 72 72 74 71 72 71 71 74
41 39 43 40 41 42 42 41 41 42 42 41 40 44 44 38 37 40 38 38 44 41 42 42 39 39 36 38 40 37 33 33 43 47 46 38 43 43 38 38 43 45 47 45 43 43 43 47
7002071 7002071 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7002071 7002071 7002071 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7029922 7029922 7029922 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7037305 7029922 7037305 7037305 7029922
BF BF BE BE BE BE DA BE BE BE BE BE BE BE BE BP BP BF BF BF CC BP CC CC CC CC BU BU BU BU CC CC BU BU BP BP BP BP BP BP CR CS CR CR CR CR CR CS
Jet Area Jet Area (1 Jet, .001") (1 Jet, .001") Cruise WOT 2.5282 2.6539 2.7332 2.8149 2.7512 2.7999 2.5737 2.7512 2.7512 2.6861 2.6861 2.8651 2.8149 2.4387 2.4387 2.6052 2.6641 2.5918 2.7143 2.7143 2.4387 2.5282 2.5737 2.5737 2.6539 2.6539 2.6138 2.4976 2.5918 2.6641 2.6704 2.6704 2.5070 2.6829 2.2973 3.0513 2.8486 2.8486 3.0513 2.9374 2.6193 2.4811 2.5659 2.3688 2.6193 2.5070 2.5070 2.5659
3.3175 3.3175 3.6545 3.5406 3.5406 3.6545 3.4283 3.5406 3.5406 3.5406 3.5406 3.6545 3.5406 3.4283 3.4283 3.2084 3.2084 3.3175 3.3175 3.3175 3.4283 3.3175 3.4283 3.4283 3.3175 3.3175 3.1008 3.1008 3.3175 3.2084 2.9947 2.9947 3.4283 3.8869 3.4283 3.6545 3.7699 3.7699 3.6545 3.5406 3.5406 3.5406 3.7699 3.4283 3.5406 3.4283 3.4283 3.7699
7042276 7042278 7043262 7043263 7043264 7043266 7043270 7043272 7043273 7043274 7044266 7044268 7044269 7044274 7044568 17054910
Pont 72 455 AT HI ALTITUDE Pont 72 400 AT HI ALTITUDE Pont 73 455 AT Pont 73 400 MT Pont 73 400 AT Pont 73 400 LATE AT Pont 73 455 S.D. AT Pont 73 455 AT HI ALTITUDE Pont 73 455 SD MT Pont 73 400 AT HI ALTITUDE Pont 74 ALL AT Pont 74 350 AT Pont 74 350 MT Pont 74 ALL HI ALTITUDE Pont 74 350 AT CALIFORNIA Pont 74 455 SD Replacement
71 72 71 71 72 72 76 70 75 72 72 72 68 72 72 75
43 46 41 43 43 45 51 41 49 45 45 43 35 45 43 49
7025200 7025201 7025220 7025221 7026200 7026201 7026202 7026203 7026204 7026205 7026210 7027200 7027201 7027210 7027211 7028207 7028208 7028209 7028212 7028216 7028217 7028218 7028219 7028229 7029202 7029203 7029207 7029215 7037200 7037201 7040207 7040503 7040503 7040507 7041204 7041205 7041205
Chev 65 396 AT EARLY Chev 65 396 MT EARLY Chev 65 396 AT LATE Chev 65 396 MT LATE Chev 66 396 AT Chev 66 396 MT Chev 66 327 AT EARLY Chev 66 327 MT Chev 66 427 AT Chev 66 427 MT Chev 66 327 AT LATE Chev 67 396/427 AT W/O A.I.R. Chev 67 396/427 MT W/O A.I.R. Chev 67 396/427 AT Chev 67 396/427 MT Chev 68 327/350 MT VETTE Chev 68 327/350 AT VETTE Chev 68 427 HIGH PERF MT VETTE Chev 68 327/350 AT Chev 68 427 HIGH PERF AT VETTE Chev 68 396 HIGH PERF MT VETTE Chev 68 396 HIGH PERF AT VETTE Chev 68 HIGH PERF MT VETTE Chev 68 HIGH PERF MT CHEVY II Chev 69 350 300HP AT VETTE Chev 69 350 300HP MT VETTE Chev 69 350 325HP VETTE Chev 69 396/427 MT Chev 67 396/427 AT A.I.R. Chev 67 396/427 MT A.I.R. Chev 70 350 FEDERAL VETTE Chev 69 350 300hp SS Camaro Chev 69 350 300hp Best Tuned Chev 70 350 CALIFORNIA VETTE Chev 71 454 AT VETTE Chev 71 454 MT VETTE Chev 71 454 MT VETTE
71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 71 66 66 67 67 66 71 71 71 76 76 74 76 77 77 77
44 41 44 44 44 41 45 43 46 41 45 44 41 44 41 46 46 45 46 47 45 47 36 36 42 38 36 45 46 41 44 44 43 44 49 49 49
7037851 7037851 7029529 7037851 7029529 7029529 7029529 7029529 7029529 7037851 7029529 7029529 7037851 7037851 7029529 7029529
CR CS CR CS DB DB BV CR BV DB DB DB DB DB DB BV
2.5070 2.4096 2.6389 2.5070 2.6193 2.4811 2.4936 2.5282 2.5321 2.4811 2.4811 2.6193 2.6696 2.4811 2.6193 2.5321
3.4283 3.5406 3.4283 3.4283 3.5406 3.5406 4.0055 3.3175 3.8869 3.5406 3.5406 3.5406 3.1008 3.5406 3.5406 3.8869
7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029922 7029922 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7036019 7037305 7029862 7029862 7036019 7029862
7031208 7031208 7031208 7031208 AX AX AK AK AX AX AK AX AX AX AX AN AN AX AN AX AX AX BG BG AN AN BG AX AX AX BA BA AK BA BG BG BG
2.4387 2.6389 2.4387 2.4387 2.4387 2.6389 2.3688 2.5070 2.2973 2.6389 2.3688 2.4387 2.6389 2.4387 2.6389 2.2973 2.2973 2.3688 2.2973 2.2242 2.3688 2.2242 2.4033 2.4033 2.1402 2.3915 2.4033 2.3688 2.2973 2.6389 3.0159 3.0159 2.8486 3.0159 2.7709 2.7709 2.7709
3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 3.4283 2.8903 2.8903 2.9947 2.9947 2.8903 3.4283 3.4283 3.4283 4.0055 4.0055 3.7699 4.0055 4.1257 4.1257 4.1257
7041212 7041213 7042203 7042216 7042217 7042902 7042903 7043200 7043201 7043202 7043203 7043212 7043213 7044206 7044207 7044208 7044209 7044210 7044211 7044506 7044507 7045200 7045210 7045211 7045213 7045216 7045222 7045223 7045228 7045229 7045504 7045583 7045586 17056206 17056207 17056210 17056211 17056226 17056506 17056507 17057203 17057210 17057211 17057228 17058203 17058210 17058211 17058228 17059203 17059210 17059211 17059228 17080201
CHEV 71 Vette 350 A/T CHEV 71 Vette 350 M/T CHEV 72 Vette Fed M/T 350 CHEV 72 Vette A/T 454 CHEV 72 Vette M/T 454 CHEV 72 Vette Fed A/T 350 CHEV 72 Vette Calif. M/T 350 CHEV 73 454 all Auto CHEV 73 454 all M/T CHEV 73 350 all Auto CHEV 73 350 all M/T CHEV 73 Vette Hi Perf. Auto CHEV 73 Vette Hi Perf. M/T CHEV 74 Vette & Nova Fed A/T CHEV 74 Vette & Nova Fed M/T CHEV 74 350 Camaro Hi Perf. A/T CHEV 74 350 Camaro Hi Perf. M/T CHEV 74 Vette 350 Hi Perf. M/T CHEV 74 Vette 350 Hi Perf. A/T CHEV 74 Vette & Nova Calif. A/T CHEV 74 Vette & Nova Calif. M/T Chev 75 454 AT Chevelle/Monte Chev 75 FEDERAL AT HIGH PERF VETTE Chev 75 FEDERAL MT HIGH PERF VETTE CHEV & GMC Truck 75-76 Non-CA, HD CHEV & GMC Truck, 75-76 Reg Chassis Chev 75 AT ALL VETTE Chev 75 FEDERAL MT VETTE Chev 75 400 FEDERAL AT Chevelle/Monte Chev 75 400 MT Truck Chev 75 350 AT CALIFORNIA CHEV & GMC Truck, 75-77 Calif. CHEV & GMC Truck, 75-77 Reg Chassis CA CHEV 76 Vette & Nova A/T CHEV 76 Vette & Nova M/T Chev 76 FEDERAL AT VETTE Chev 76 FEDERAL MT VETTE Chev 76 FEDERAL AT A/C VETTE CHEV 76 Vette & Nova A/T Calif CHEV 76 Vette & Nova M/T Calif Chev 77 FEDERAL MT NON-A/C VETTE Chev 77 HIGH PERF NON-A/C VETTE Chev 77 HIGH PERF A/C & NON-A/C VETTE Chev 77 FEDERAL A/C VETTE CHEV 78 FED VETTE A/C & NON-A/C 4SPD CHEV 78 FED VETTE NON A/C AUTO CHEV 78 FED VETTE H.P. A/C & NON 4SPD CHEV 78 FED VETTE H.P. A/C AUTO CHEV 79 FED VETTE A/C & NON 4-SPD CHEV 79 VETTE L-82 H.P. NON-A/C AUTO CHEV 79 VETTE L-82 H.P. A/C & NON 4SPD CHEV 79 VETTE L-82 H.P. A/C AUTO CHEV 80
74 74 74 77 77 74 74 77 77 73 73 74 74 75 75 75 75 75 75 75 75 76 72 72 68 68 72 72 71 69 72 73 73 77 77 77 77 77 77 77 77 77 77 77 77
44 44 45 49 45 45 45 50 48 44 44 44 44 46 46 43 43 43 43 46 46 43 44 44 32 32 46 46 47 36 46 42 42 48 48 51 51 51 48 48 52 53 53 53 52
7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7029862 7041477 7041459 7041459 7029862 7029862 7041459 7041459 17052057 7029862 7037851 7029862 7029862 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459
AR AR DA CM CM DA DA DA DA DA DA DA DA CH CH DA DA DA DA CH CH CJ CH CH CP CP CH CH DL DL CH CP CP CH CH CH CH CH CH CH CH CH CH CH CH
2.7803 2.7803 2.7104 2.7709 3.0662 2.7104 2.7104 2.6931 2.8471 2.6649 2.6649 2.7803 2.7803 2.7560 2.7560 2.9657 2.9657 2.9657 2.9657 2.7560 2.7560 3.0843 2.5510 2.5510 2.8274 2.8274 2.4096 2.4096 2.2242 2.7214 2.4096 2.7999 2.7999 2.8471 2.8471 2.6138 2.6138 2.6138 2.8471 2.8471 2.5329 2.4504 2.4504 2.4504 2.5329
3.7699 3.7699 3.7699 4.1257 4.1257 3.7699 3.7699 4.1257 4.1257 3.6545 3.6545 3.7699 3.7699 3.8869 3.8869 3.8869 3.8869 3.8869 3.8869 3.8869 3.8869 4.0055 3.5406 3.5406 3.1008 3.1008 3.5406 3.5406 3.4283 3.2084 3.5406 3.6545 3.6545 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257 4.1257
77 77
53 53
7041459 7041459
CH CH
2.4504 2.4504
4.1257 4.1257
77 72 77 77
53 40 53 53
7041459 7041459 7041459 7041459
CH CH CH CH
2.4504 2.8149 2.4504 2.4504
4.1257 3.5406 4.1257 4.1257
77 71
53 48
7041459 7041459
CH DR
2.4504 2.1496
4.1257 3.4283
17080202 17080204 17080205 17080206 17080207 17080212 17080213 17080215 17080224
CHEV 80 CHEV 80 CHEV 80 CHEV 80 Chev 80 VETTE M/T CHEV 80 CHEV 80 CHEV 80 CHEV 80
71 71 72 72 71 72 72 72 72
42 42 51 51 42 52 50 50 48
7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459 7041459
CH CH DP DP CH 7048992 DP DP DR
2.5737 2.5737 2.0287 2.0287 2.5737 1.9478 2.1080 2.1080 2.2619
3.4283 3.4283 3.5406 3.5406 3.4283 3.5406 3.5406 3.5406 3.5406
Figure 2: Metering Areas of Jet & Rod Combinations Jet
Jet Area
0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060 0.060
0.00282743
0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061 0.061
0.00292247
0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743 0.00282743
0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247 0.00292247
Rod
Rod Area Resulting Area
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.0022965
0.00070686
0.00212058
0.00075477
0.00207267
0.00080425
0.00202319
0.0008553
0.00197213
0.00090792
0.00191951
0.00096211
0.00186532
0.00101788
0.00180956
0.00107521
0.00175222
0.00113411
0.00169332
0.00119459
0.00163284
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00125664
0.0015708
0.00132025
0.00150718
0.00138544
0.00144199
0.0014522
0.00137523
0.00152053
0.0013069
0.00159043
0.001237
0.0016619
0.00116553
0.00173494
0.00109249
0.00180956
0.00101788
0.00188574
0.00094169
0.0019635
0.00086394
0.00053093
0.00239154
0.00070686
0.00221561
0.00075477
0.0021677
0.00080425
0.00211822
0.0008553
0.00206717
0.00090792
0.00201455
0.00096211
0.00196035
0.00101788
0.00190459
0.00107521
0.00184726
0.00113411
0.00178835
0.00119459
0.00172788
0.00125664
0.00166583
0.00132025
0.00160221
0.00138544
0.00153702
0.0014522
0.00147027
0.00152053
0.00140194
0.00159043
0.00133204
0.0016619
0.00126056
0.00173494
0.00118752
0.00180956
0.00111291
0.00188574
0.00103673
0.0019635
0.00095897 0
0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062 0.062
0.00301907
0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063 0.063
0.00311725
0.064 0.064 0.064 0.064 0.064 0.064
0.00321699
0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907 0.00301907
0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725 0.00311725
0.00321699 0.00321699 0.00321699 0.00321699 0.00321699
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00248814
0.00070686
0.00231221
0.00075477
0.0022643
0.00080425
0.00221482
0.0008553
0.00216377
0.00090792
0.00211115
0.00096211
0.00205696
0.00101788
0.00200119
0.00107521
0.00194386
0.00113411
0.00188496
0.00119459
0.00182448
0.00125664
0.00176243
0.00132025
0.00169882
0.00138544
0.00163363
0.0014522
0.00156687
0.00152053
0.00149854
0.00159043
0.00142864
0.0016619
0.00135717
0.00173494
0.00128413
0.00180956
0.00120951
0.00188574
0.00113333
0.0019635
0.00105558
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00258632
0.00070686
0.00241039
0.00075477
0.00236248
0.026 0.030 0.031 0.032 0.033 0.034
0.00080425
0.002313
0.0008553
0.00226195
0.00090792
0.00220933
0.00096211
0.00215513
0.00101788
0.00209937
0.00107521
0.00204204
0.00113411
0.00198313
0.00119459
0.00192265
0.00125664
0.00186061
0.00132025
0.00179699
0.00138544
0.0017318
0.0014522
0.00166504
0.00152053
0.00159671
0.00159043
0.00152681
0.0016619
0.00145534
0.00173494
0.0013823
0.00180956
0.00130769
0.00188574
0.0012315
0.0019635
0.00115375
0.00053093
0.00268606
0.00070686
0.00251013
0.00075477
0.00246222
0.00080425
0.00241274
0.0008553
0.00236169
0.00090792
0.00230907
0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064 0.064
0.00321699
0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065 0.065
0.00331831
0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066
0.00342119
0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699 0.00321699
0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831 0.00331831
0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119
0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00096211
0.00225488
0.00101788
0.00219911
0.00107521
0.00214178
0.00113411
0.00208288
0.00119459
0.0020224
0.00125664
0.00196035
0.00132025
0.00189674
0.00138544
0.00183155
0.0014522
0.00176479
0.00152053
0.00169646
0.00159043
0.00162656
0.0016619
0.00155509
0.00173494
0.00148205
0.00180956
0.00140743
0.00188574
0.00133125
0.0019635
0.0012535
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00278738
0.00070686
0.00261145
0.00075477
0.00256354
0.00080425
0.00251406
0.0008553
0.00246301
0.00090792
0.00241039
0.00096211
0.00235619
0.00101788
0.00230043
0.00107521
0.0022431
0.00113411
0.00218419
0.00119459
0.00212372
0.00125664
0.00206167
0.00132025
0.00199805
0.00138544
0.00193286
0.0014522
0.00186611
0.00152053
0.00179778
0.00159043
0.00172788
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040
0.0016619
0.0016564
0.00173494
0.00158336
0.00180956
0.00150875
0.00188574
0.00143257
0.0019635
0.00135481
0.00053093
0.00289027
0.00070686
0.00271434
0.00075477
0.00266643
0.00080425
0.00261695
0.0008553
0.0025659
0.00090792
0.00251327
0.00096211
0.00245908
0.00101788
0.00240332
0.00107521
0.00234598
0.00113411
0.00228708
0.00119459
0.0022266
0.00125664
0.00216456
0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066 0.066
0.00342119
0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067 0.067
0.00352565
0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068 0.068
0.00363168
0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119 0.00342119
0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565 0.00352565
0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168 0.00363168
0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00132025
0.00210094
0.00138544
0.00203575
0.0014522
0.00196899
0.00152053
0.00190066
0.00159043
0.00183076
0.0016619
0.00175929
0.00173494
0.00168625
0.00180956
0.00161164
0.00188574
0.00153545
0.0019635
0.0014577
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00299472
0.00070686
0.00281879
0.00075477
0.00277088
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046
0.00080425
0.0027214
0.0008553
0.00267035
0.00090792
0.00261773
0.00096211
0.00256354
0.00101788
0.00250778
0.00107521
0.00245044
0.00113411
0.00239154
0.00119459
0.00233106
0.00125664
0.00226902
0.00132025
0.0022054
0.00138544
0.00214021
0.0014522
0.00207345
0.00152053
0.00200512
0.00159043
0.00193522
0.0016619
0.00186375
0.00173494
0.00179071
0.00180956
0.00171609
0.00188574
0.00163991
0.0019635
0.00156216
0.00053093
0.00310075
0.00070686
0.00292482
0.00075477
0.00287691
0.00080425
0.00282743
0.0008553
0.00277638
0.00090792
0.00272376
0.00096211
0.00266957
0.00101788
0.00261381
0.00107521
0.00255647
0.00113411
0.00249757
0.00119459
0.00243709
0.00125664
0.00237504
0.00132025
0.00231143
0.00138544
0.00224624
0.0014522
0.00217948
0.00152053
0.00211115
0.00159043
0.00204125
0.0016619
0.00196978
0.068 0.068 0.068 0.068
0.00363168
0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069 0.069
0.00373928
0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070
0.00384845
0.071
0.00363168 0.00363168 0.00363168
0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928
0.047 0.048 0.049 0.050
0.00173494
0.00189674
0.00180956
0.00182212
0.00188574
0.00174594
0.0019635
0.00166819
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00320835
0.00070686
0.00303242
0.00075477
0.00298451
0.00080425
0.00293503
0.0008553
0.00288398
0.00090792
0.00283136
0.00096211
0.00277717
0.00101788
0.0027214
0.00107521
0.00266407
0.00113411
0.00260517
0.00119459
0.00254469
0.00125664
0.00248264
0.00132025
0.00241903
0.00138544
0.00235384
0.0014522
0.00228708
0.00152053
0.00221875
0.00159043
0.00214885
0.0016619
0.00207738
0.00173494
0.00200434
0.00180956
0.00192972
0.00188574
0.00185354
0.0019635
0.00177579
0.00053093
0.00331752
0.00070686
0.00314159
0.00075477
0.00309368
0.00384845
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00395919
0.026
0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928 0.00373928
0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845 0.00384845
0.00080425
0.0030442
0.0008553
0.00299315
0.00090792
0.00294053
0.00096211
0.00288634
0.00101788
0.00283057
0.00107521
0.00277324
0.00113411
0.00271434
0.00119459
0.00265386
0.00125664
0.00259181
0.00132025
0.0025282
0.00138544
0.00246301
0.0014522
0.00239625
0.00152053
0.00232792
0.00159043
0.00225802
0.0016619
0.00218655
0.00173494
0.00211351
0.00180956
0.00203889
0.00188574
0.00196271
0.0019635
0.00188496
0.00053093
0.00342826
0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071
0.00395919
0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072 0.072
0.0040715
0.073 0.073 0.073 0.073 0.073 0.073 0.073
0.00418539
0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919 0.00395919
0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715 0.0040715
0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539
0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00070686
0.00325233
0.00075477
0.00320442
0.00080425
0.00315494
0.0008553
0.00310389
0.00090792
0.00305127
0.00096211
0.00299708
0.00101788
0.00294132
0.00107521
0.00288398
0.00113411
0.00282508
0.00119459
0.0027646
0.00125664
0.00270256
0.00132025
0.00263894
0.00138544
0.00257375
0.0014522
0.00250699
0.00152053
0.00243866
0.00159043
0.00236876
0.0016619
0.00229729
0.00173494
0.00222425
0.00180956
0.00214963
0.00188574
0.00207345
0.0019635
0.0019957
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00354057
0.00070686
0.00336465
0.00075477
0.00331674
0.00080425
0.00326726
0.026 0.030 0.031 0.032 0.033 0.034 0.035
0.0008553
0.00321621
0.00090792
0.00316358
0.00096211
0.00310939
0.00101788
0.00305363
0.00107521
0.00299629
0.00113411
0.00293739
0.00119459
0.00287691
0.00125664
0.00281487
0.00132025
0.00275125
0.00138544
0.00268606
0.0014522
0.0026193
0.00152053
0.00255097
0.00159043
0.00248107
0.0016619
0.0024096
0.00173494
0.00233656
0.00180956
0.00226195
0.00188574
0.00218576
0.0019635
0.00210801
0.00053093
0.00365446
0.00070686
0.00347853
0.00075477
0.00343062
0.00080425
0.00338114
0.0008553
0.00333009
0.00090792
0.00327747
0.00096211
0.00322327
0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073 0.073
0.00418539
0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074 0.074
0.00430084
0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075
0.00441786
0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539 0.00418539
0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084 0.00430084
0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786
0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00101788
0.00316751
0.00107521
0.00311018
0.00113411
0.00305127
0.00119459
0.0029908
0.00125664
0.00292875
0.00132025
0.00286513
0.00138544
0.00279994
0.0014522
0.00273319
0.00152053
0.00266486
0.00159043
0.00259496
0.0016619
0.00252348
0.00173494
0.00245044
0.00180956
0.00237583
0.00188574
0.00229965
0.0019635
0.00222189
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00376991
0.00070686
0.00359398
0.00075477
0.00354607
0.00080425
0.00349659
0.0008553
0.00344554
0.00090792
0.00339292
0.00096211
0.00333873
0.00101788
0.00328296
0.00107521
0.00322563
0.00113411
0.00316673
0.00119459
0.00310625
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041
0.00125664
0.0030442
0.00132025
0.00298059
0.00138544
0.0029154
0.0014522
0.00284864
0.00152053
0.00278031
0.00159043
0.00271041
0.0016619
0.00263894
0.00173494
0.0025659
0.00180956
0.00249128
0.00188574
0.0024151
0.0019635
0.00233734
0.00053093
0.00388694
0.00070686
0.00371101
0.00075477
0.0036631
0.00080425
0.00361362
0.0008553
0.00356257
0.00090792
0.00350994
0.00096211
0.00345575
0.00101788
0.00339999
0.00107521
0.00334265
0.00113411
0.00328375
0.00119459
0.00322327
0.00125664
0.00316123
0.00132025
0.00309761
0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075 0.075
0.00441786
0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076 0.076
0.00453646
0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077
0.00465663
0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786 0.00441786
0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646 0.00453646
0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663 0.00465663
0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00138544
0.00303242
0.0014522
0.00296566
0.00152053
0.00289733
0.00159043
0.00282743
0.0016619
0.00275596
0.00173494
0.00268292
0.00180956
0.00260831
0.00188574
0.00253212
0.0019635
0.00245437
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00400553
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047
0.00053093
0.0041257
0.00070686
0.00394977
0.00075477
0.00390186
0.00080425
0.00385238
0.00070686
0.0038296
0.00075477
0.00378169
0.00080425
0.00373221
0.0008553
0.00368116
0.00090792
0.00362854
0.00096211
0.00357435
0.00101788
0.00351858
0.00107521
0.00346125
0.00113411
0.00340234
0.00119459
0.00334187
0.00125664
0.00327982
0.00132025
0.00321621
0.00138544
0.00315102
0.0014522
0.00308426
0.00152053
0.00301593
0.00159043
0.00294603
0.0016619
0.00287456
0.00173494
0.00280152
0.00180956
0.0027269
0.00188574
0.00265072
0.0019635
0.00257296
0.0008553
0.00380133
0.00090792
0.00374871
0.00096211
0.00369451
0.00101788
0.00363875
0.00107521
0.00358142
0.00113411
0.00352251
0.00119459
0.00346204
0.00125664
0.00339999
0.00132025
0.00333637
0.00138544
0.00327118
0.0014522
0.00320442
0.00152053
0.00313609
0.00159043
0.00306619
0.0016619
0.00299472
0.00173494
0.00292168
0.077 0.077 0.077
0.00465663
0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078 0.078
0.00477836
0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079 0.079
0.00490167
0.080 0.080
0.00502655
0.00465663 0.00465663
0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836 0.00477836
0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167 0.00490167
0.00502655
0.048 0.049 0.050
0.00180956
0.00284707
0.00188574
0.00277088
0.0019635
0.00269313
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00424743
0.00070686
0.0040715
0.00075477
0.00402359
0.00080425
0.00397411
0.0008553
0.00392306
0.00090792
0.00387044
0.00096211
0.00381625
0.00101788
0.00376049
0.00107521
0.00370315
0.00113411
0.00364425
0.00119459
0.00358377
0.00125664
0.00352173
0.00132025
0.00345811
0.00138544
0.00339292
0.0014522
0.00332616
0.00152053
0.00325783
0.00159043
0.00318793
0.0016619
0.00311646
0.00173494
0.00304342
0.00180956
0.00296881
0.00188574
0.00289262
0.0019635
0.00281487
0.026 0.030 0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00053093
0.00437074
0.00070686
0.00419481
0.026 0.030
0.00075477
0.0041469
0.00080425
0.00409742
0.0008553
0.00404637
0.00090792
0.00399375
0.00096211
0.00393956
0.00101788
0.00388379
0.00107521
0.00382646
0.00113411
0.00376755
0.00119459
0.00370708
0.00125664
0.00364503
0.00132025
0.00358142
0.00138544
0.00351623
0.0014522
0.00344947
0.00152053
0.00338114
0.00159043
0.00331124
0.0016619
0.00323977
0.00173494
0.00316673
0.00180956
0.00309211
0.00188574
0.00301593
0.0019635
0.00293817
0.00053093
0.00449562
0.00070686
0.00431969
0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080 0.080
0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655 0.00502655
0.031 0.032 0.033 0.034 0.035 0.036 0.037 0.038 0.039 0.040 0.041 0.042 0.043 0.044 0.045 0.046 0.047 0.048 0.049 0.050
0.00075477
0.00427178
0.00080425
0.0042223
0.0008553
0.00417125
0.00090792
0.00411863
0.00096211
0.00406444
0.00101788
0.00400867
0.00107521
0.00395134
0.00113411
0.00389243
0.00119459
0.00383196
0.00125664
0.00376991
0.00132025
0.00370629
0.00138544
0.00364111
0.0014522
0.00357435
0.00152053
0.00350602
0.00159043
0.00343612
0.0016619
0.00336465
0.00173494
0.0032916
0.00180956
0.00321699
0.00188574
0.00314081
0.0019635
0.00306305
Figure 3: Secondary Metering Rods Listed Rich to Lean
Code BV CB CC DC BY CF DG AX BB BF BG BH BJ BK BM BP BW CA CJ CM CS BE BL BN CE CY DA AD AH AU CK CV BU CR AJ AK AL AP AR AV AY AZ
P/N 7040724 7042335 7042356 7047816 7040856 7044775 7048890 7033549 7034335 7034400 7034822 7035916 7036077 7037295 7037744 7038034 7040767 7042304 7045780 7045840 7045924 7034377 7037733 7036671 7043771 7046004 7046010 7033772 7033812 7033655 7045781 7045984 7040725 7045923 7033628 7033104 7033680 7033981 7033171 7033182 7033830 7033889
Dia of Tip 0.0300 0.0300 0.0303 0.0303 0.0320 0.0340 0.0340 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0400 0.0413 0.0413 0.0413 0.0413 0.0443 0.0443 0.0450 0.0530 0.0530 0.0530 0.0530 0.0550 0.0550 0.0570 0.0570 0.0570 0.0570 0.0570 0.0570 0.0570 0.0570
Tip Length S S M M M M M S S S M M S S M S M M S M S S S S L M M S M L L L S S M S S M S M L L
BA BZ CD CH CN CP CX BD BC BT AT CL DL AN BX DB AS CG CT DE BR AW BS CZ DD DF DH DK DM DN DP DR DS DU
7034337 7042300 7042719 7045779 7045841 7045842 7045985 7034365 7034300 7040601 7033658 7045782 7048892 7034320 7040797 7047806 7045778 7045778 7045983 7048092 7038910 7033194 7038911 7045986 7048091 7048512 7048992 7048919 17050221 17053703 17053531 17053659 17056618 17059952
0.0570 0.0570 0.0570 0.0570 0.0570 0.0570 0.0570 0.0580 0.0584 0.0600 0.0670 0.0670 0.0690 0.0700 0.0700 0.0700 0.0777 0.0777 0.0777 0.0877 0.0900 0.0908 0.0950 0.0950 0.1050
S L L S S S L M S M L L S S S S M M M M L M L L L
Technical Procedure #1: To pop the top off a Q-Jet, proceed as follows: 1. Remove the air cleaner stud. 2. Using a hammer and a small pin punch or a small finish nail, tap the roll pin holding the accelerator pump lever to the top of the carb in towards the choke horn wall. Don't tap the roll pin all the way up against the wall - leave just a slight gap so you can later get a screwdriver blade in behind it to pry it back again. Remove the accel pump lever. 3. Remove the single screw holding the secondary rod hanger to the top of the carb and remove the hanger with the secondary rods. 4. If you have a later-model Q-Jet with a choke vacuum break diaphragm that is attached to the passsenger side of the carb with two screws up high, remove the two screws and remove the vacuum break and its connecting rod. If your vacuum break is pressed into a bracket that is not attached with 2 screws up high, leave it alone. 5. Remove the choke connecting rod. There are 2 types: One type has a clip holding it to the choke lever. Remove the clip, disengage the rod from the upper lever, then twist/rotate the rod to disengage it from the lower lever inside the carb. Later model carbs have a single screw holding the upper lever to the choke shaft. On this type, remove the screw, remove the lever, and remove the choke rod by twisting/rotating it to release it from the lower lever inside the carb. 6. Remove the (2) 1/2" head bolts at the front of the carb. 7. Remove the 9 top attach screws: Two long screws in the very back; a screw on either side of the secondary airvalves; two screws just forward of the secondary airvalves; two screws just inside the choke air horn right at each primary discharge nozzle, and a single screw center front. If the carb has the stock screws in it, the two screws inside the air horn are designed to be too big to drop down into the intake manifold. But many aftermerket screws can, in fact, drop through the carb and go into the intake. Once you have loosned these two screws, use a pair of needle nosed pliers to carefully lift them out and make sure they don't drop. 8. Lift the top of the carb straight up until it clears the accelerator pump and until the air bleed tubes clear the gasket. If you have a non-removable vacuum break diaphragm, cock the top over to the side to disengage the secondary airvalve rod. 9. Remove the gasket by carefully freeing it from the power piston/primary metering rod hanger. 10. Remove the accelerator pump. 11. Remove the power piston/primary metering rod hanger by pushing it down against its spring pressure and "flicking" it off your fingernail so it pops up. A couple of flicks will disengage the locking collar from the casting, and the assembly can be removed. 12. Remove the phenolic float bowl filler. 13. Remove the float and needle as an assembly. 14. Remove the main jets. The rods and the jets are stamped with their sizes. Only trick for re-assembly: 1. When installing the power piston, take care to fish around until the rods drop down into the jets and the power piston works smoothly. Gently push the piston nylon locking collar back into the carb casting. I've seen people not get the rods into the jets, and simply smash the top of the carb down onto the piston/rod assembly. Obviously, this will bend the rods. Once you have the top back on, installing the choke linkage rod is considered the only "tricky" part. There is a short lever arm down inside the carb, and this arm has a hole in its end. This arm is very easy to see when you have the top off the carb, so I recommend that novices take a look at it and its orientation/function while they have the top off the carb. With the top off, take the choke rod and practice installing/engaging it in this lower lever until you get the knack of rotating the rod slightly to engage it in the hole in the lever. Once you have the top back on (taking care not to overtighten screws and bolts), activate the choke linkage on the outside of the carb to move this lever arm to its furthest "up" position. You can just barely see it if you look down the carb. Now, insert the choke rod down into the carb, with the rod rotated slightly. Engage the hole in the lever arm at this angle, and once you've hooked the arm, rotate the rod to fully engage it. Install the accelerator pump lever to the top of the carb. Insert a finish nail or a small pin punch through the roll pin hole to assure that it's aligned, and then use a small screwdriver to pry the roll pin back through the lever. Install the secondary metering rods with the hanger. NOTE: If you're going to be doing several jet changes, you do not need to attach the choke linkage rod to run the car. Leave the rod off until you're complete.
Technical Procedure #2 To adjust a Q-Jet with an adjustable Power Piston, proceed as follows: The Q-Jet uses a power piston with metering rods to lean out the fuel mixture at cruise and at idle, and to richen up the mixture at wide open throttle (WOT). When engine vacuum is high, the power piston is pulled down into the carb against spring pressure, and this inserts the “fat” part of the primary metering rods into the jets for a lean, crisp, economical fuel mixture. When engine vacuum is lost, such as occurs under high power settings, the piston pops up from the spring pressure, and the “skinny” part of the primary rod is all that remains in the jet. This increases the metering area of the jet and richens up the fuel mixture for good power and performance. Late ‘70s Q-Jets have an externally-accessible adjustment screw (through a small hole in the carb air horn) for adjustment of the power piston height. Many people refer to this as the “mixture screw” on a Q-Jet. Over the years, people have screwed these adjustment screws every way possible in an attempt to “tune” the carbs, and I now frequently see people asking about what the “spec” is for this adjustment. Fact is, there is none. But here’s how you can get your carb set up so it’ll run right again. You will need to take the air horn (the “top” of the carb) off in order to get this set up properly. See “Technical Procedure #1” this paper for the step-by-step on doing this. The adjustment screw for the power piston height is located down inside a small bore adjacent to the power piston. You can turn the flat adjustment screw with a pair of needle nosed pliers. Note that the adjustment screw only adjusts and limits how far DOWN the power piston can go. There is no “up” limit on the piston that is adjustable. In other words, the screw sets the maximum depth that the rods engage into the jets at cruise and at idle. The intent of the adjustment is to assure that the “fat” part of the rod is fully inserted into the metering orifice of the jet under these conditions. If it’s set too shallow, with the skinny power tip portion of the rod in the jet, the mixture will be too rich. If it’s set too deep, the mixture will stay too lean as the engine gets into its power curve. We want to set the height so that the rod is fully inserted in the jet at cruise, but not set too deep. To do this, you need a pair of calipers. Dial calipers are nice, but I use an old pair of vernier calipers. With the top off the carb, remove the power piston, remove one of the main metering rods, and remove one main metering jet. Lay the jet and the rod side by side on your workbench, and align the rod with the jet such that the top “step” in the rod (the step-up from the fat metering part of the rod to the main shaft of the rod) is aligned next to the bottom of the upper “lip” of the main jet (see Figure bellow) Note that I have given a “range” for this measurement: lining the rod up with the lower edge of the lip is the “max engagement” depth. Lining it up at the mid-point of the lip is the “min engagement” depth. If the rod is in this range, the resultant jetting will be correct. Now, measure the distance from the top surface of the jet to the very top of the rod. Record this number. Re-install the jet, the rod, and the power piston into the carb. Press down on the power piston until it seats. Using the calipers, measure the distance from the tip top of the main metering rod to the top surface of the jet and adjust the adjustment screw until you obtain the measurement you recorded earlier. This measurement assures that the rod is fully inserted in the jet at cruise, and this will give you correct, excellent performance.
